Our brain imaging studies of subjects with Williams Syndrome (WMS) have attempted to identify anatomical distinctions between WMS subjects and different control groups that might shed some light on the neural bases for their distinctive behavioral profiles. our results suggest that structures in the basal ganglia and diencephalon are preserved in size relative to cortical structures in Down Syndrome (DNS) subjects. Furthermore, frontal cortical structures are disproportionately affected. In contrast, WMS subjects exhibit relatively normal frontal and limbic cortices, in addition to cerebellar size preservation. These observations serve as the basis for the following working hypotheses about relationships between behavioral and neuroanatomical features of WMS and DNS: It may be that relative preservation of superior temporal and related frontal cortical and insular structures, and possibly neocerebellar preservation, underly the sparing of linguistic functions in WMS. Similarly, preserved affective function and social abilities may emerge as a result of sparing of structures such as amygdala and related limbic structures, and mesial and orbitofrontal cortices. The disproportionate impairment on visuo-motor tasks may be related to particular vulnerability of parieto-occipital cortical structures in WMS. The goal of the proposed anatomical studies is to garner evidence relevant to these working hypotheses, and to extend our anatomical findings by using a higher-resolution imaging protocol and obtaining more specific anatomical measures in larger groups of WMS subjects and controls. With a standardized MR imaging protocol and brain morphometric analysis, we propose to study 100 WMS subjects, 50 IQ-matched DNS subjects, and 30 normal age-matched controls over five years.